The present invention relates to the forging of workpieces and more specifically to the providing of a forging lubricant which prevents oxidation during heating of a workpiece and provides good lubrication during forging of the workpiece.
It has been suggested that forging lubricants having various compositions could be applied to a workpiece by brushing, spraying, dipping in the manner disclosed in U.S. Pat. No. 4,281,528. In addition, the electrophoretic deposition of a forging lubricant onto the surface of a workpiece is disclosed in U.S. Pat. No. 4,318,792. These forging lubricants may be formed of glass and promote good surface quality, metal flow, dimensional accuracy, grain structure and mechanical properties in the forged part.
When a workpiece is to be forged, it is preheated to the forging temperature, approximately 1700.degree. F. for titanium alloy aircraft engine components. During preheating, the glass should quickly fuse at a relatively low temperature to form a coherent uniform layer over the workpiece to prevent oxidation of the workpiece and the development of alpha case.
When the workpiece is placed in the dies, the forging lubricant should act as a thermal barrier between the workpiece and the dies to retard the transfer of heat from the workpiece to the dies with a resulting chilling of the workpiece. However, perhaps most importantly, the forging lubricant should possess sufficient viscosity at forging temperatures to enable it to act as a hydrodynamic lubricant.
The frictional restraint to metal flow during the forging process is directly proportional to the viscosity of the layer of glass lubricant. However, as viscosity is increased, the frictional restraint to metal flow increases and reduces the effectiveness of the lubricant. Therefore, excessive lubricant viscosity leads to an increase in the overall frictional factors in the forging operation.
If the viscosity of the lubricant is too low, then a very thin layer of lubricant is provided between the die and the workpiece. This thin layer of lubricant allows a plastic shearing of the forged part surfaces by the dies. In addition, if the viscosity is too low, the lubricant is squeezed out from between the workpiece and the dies. Of course, this destroys the hydrodynamic layer and prevents the lubricant from performing its intended function.